The availability of clean water for use worldwide is shrinking with increasing population and expansion in urbanization and industrialization. While over 70% of the world is water, only about 2.5% is fresh water. To meet the increasing demands for usable fresh water, technologies such as seawater and brackish water desalination have been developed. Because seawater and brackish water contain large amounts of impurities, including salts, minerals, and other dissolved ions, the water must be treated before industrial, agricultural or home use.
Reverse osmosis (RO) membrane desalination uses membrane technology to transform seawater and brackish water into fresh water for drinking, irrigation, and industrial applications by separating dissolved substances, such as salts, minerals and ions, from the water. Reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. This is the reverse of an osmosis process, which is the natural movement of solvent from an area of low solute concentration through a membrane to an area of high solute concentration with no external pressure applied. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute. RO desalination processes require substantially less energy than do thermal desalination processes, e.g., multi-stage flash; thus, reverse osmosis membrane technology is increasingly used to produce fresh water from seawater or brackish water.
The membranes used for RO are composite membranes made up of a porous support and a thin polyamide layer formed on the support. Typically, the polyamide layer is formed by interfacial polymerization of a polyfunctional amine and a polyfunctional acid halide, creating a dense barrier layer in the polymer matrix where most separation occurs. The membranes are designed to allow only water to pass through this dense layer while preventing the passage of solutes such as salts, minerals and ions. The reverse osmosis process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2-17 bar (30-250 psi) for fresh and brackish water, and 40-70 bar (600-1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome.
During the interfacial polymerization reaction between the polyfunctional amine and the polyfunctional acid halide, hydrolysis of the polyfunctional acid halide readily occurs. Trimesoyl chloride (TMC) is a polyfunctional acid halide commonly used in the formation of thin film composite (TFC) membranes that has three acyl halide groups that readily hydrolyze in air to mono-hydrolyzed TMC (a molecule of trimesoyl chloride in which one of the —Cl groups has been replaced with an —OH group), with di-hydrolyzed trimesoyl chloride and tri-hydrolyzed trimesoyl chloride (i.e., trimesic acid) often present at low levels. The hydrolysis products of TMC, when incorporated into thin film composite membranes for RO applications, can yield membranes with high flux but can also negatively affect salt rejection characteristics as compared to a membrane made with purified TMC. TMC can be purified prior to use, but unless stringent humidity controls are in place, hydrolysis will continue throughout the interfacial polymerization process.
Thus, there remains a need to develop RO membranes, including TFC membranes, that achieve high rejection characteristics, such as high salt rejection. Also desired are methods for making the membranes. Accordingly, it is among the objects herein to provide RO membranes, including TFC membranes, that achieve high rejection values, particularly with respect to higher concentration salts, and methods for making the membranes.